There are now essentially three ways of configuring comparators
when creating a Dbi.

**null comparator**
LMDB will use its own comparator & CursorIterable will call down
to mdb_cmp for comparisons between the current cursor key and the
range start/stop key.

**provided comparator**
LMDB will use its own comparator & CursorIterable will use the
provided comparator for comparisons between the current cursor
key and the range start/stop key.

**provided comparator with nativeCb==true**
LMDB will call back to java for all comparator duties.
CursorIterable will use the same provided comparator for
comparisons between the current cursor key and the range
start/stop key.

The methods `getSignedComparator()` and `getUnsignedComparator()`
have been made public so users of this library can access them.